The addition of Mg2+ is found to turn on the fluorescence response of a molecular probe, 3,3'-diformyl-1,1'-bi-2-naphthol, toward chiral amino acids with high enantioselectivity. It is further found that the enantioselective fluorescence responses of the molecular probe in the presence of Mg2+ toward certain amino acids are the opposite of those in the presence of Zn2+, that is, using Mg2+ with an l-amino acid generates much greater fluorescence enhancement than with the corresponding d-amino acid, but using Zn2+ with the d-amino acid gives much greater fluorescence than with the l-enantiomer. Thus, simply changing the metal cation additive allows the chirality sense of the fluorescence-based molecular recognition to be easily regulated.